The present invention relates to disk pack servo systems, and particularly to an eddy current brake servo system for use with rotating apparatus having eddy current dynamic brakes.
Disk pack apparatus utilize record/readout heads which must be stepped radially across the rotating disk surface to define consecutive concentric record tracks. Obviously, it takes a finite time for the head to "seek" the next track location, either in the record or replay mode. In addition, the heads must step, or seek, at a specific positional location, e.g., sector zero, on the disk pack, and then settle down sufficiently to allow recording or replay. The delay experienced during the rotation of the disk pack while awaiting the specific positional location where the heads are allowed to seek the next track, is termed the "latency" of the disk pack.
It follows that a single disk pack is incapable of accurately loading the high speed data due to the head seek time and the inherent latency of the disk pack apparatus, i.e., the data cannot wait, for example, for the disk pack to rotate to sector zero.
One way of overcoming the problem is to provide a single disk pack apparatus with very large buffer means to buffer the incoming high speed data to compensate for the latency on the disk pack. Such a system is relatively unwieldly, complex, expensive and generally undesirable.
A second means for providing the transfer of high speed data at very high data rates such as when loading satellite data, utilizes a parallel transfer disk (PTD) system, employing a pair of disk pack apparatus. In such a PTD system, high speed data is loaded into, or read from, one disk pack while the heads seek on the second disk pack. The incoming data is then switched to the second disk pack while the heads seek on the first disk pack. This alternate switching between disk packs allows continuous loading, or readout, of high speed data without interruption.
However, in such use of a PTD system, it is imperative that each disk pack is in a precisely correct rotational position, i.e., that the disk packs are locked together to allow switching between them at precisely the right rotational position, for example, at sector zero. Thus, a servo system must be used to precisely control the speed and the positional phase of both disk packs, and to lock the two packs together.
Servo systems are available which servo a motor, such as the large, 1 horsepower, AC motor used in disk pack apparatus, which employ a DC to AC controller to control the frequency to the motor. Such a servo system requires on the order of 11/2 horsepower to control the motor, and the servo system itself must handle the relatively large wattage requirements. Such a servo system generally is not efficient, is relatively expensive, and is relatively unreliable.
Eddy current dynamic brakes are commonly used in rotating disk packs, and the like, to provide for quickly stopping the disk pack when the drive is turned off. The brake operates by the application of current through the brake's electromagnetic coil, which causes the generation of flux lines through an integral brake disk attached to the disk pack spindle. These flux lines induce eddy currents which, in turn, develop their own magnetic field. This magnetic field opposes that of the electromagnetic coil, and these two forces opposing each other develop the braking action. The amount of braking force is directly proportional to the rotational speed and flux produced by the electromagnetic coil, whereby the braking reflects this condition.
As may be seen, the dynamic brake is energized only during that period of time when it is desirable to stop the disk pack rotation.